ABSTRACT: The incidence and mortality of prostate cancer (PrCa) is a significant clinical problem. One of the few PrCa risk factors is a positive family history of the disease, suggesting a heritable component to the disease. The HOXB13(G84E) (c251G-->A) mutation has been identified as one of the only single nucleotide polymorphisms (SNP) that can substantially increase a man?s odds ratio for PrCa by levels comparable to the increased risk associated with positive family history. The mechanism of how G84E increases disease risk remains yet unknown. This and other clinically relevant mutations in HOXB13 sit in the MEIS-Interacting-Domains of HOXB13 and thus likely alter MEIS-HOXB13 interactions that provide DNA-binding specificity and protein recruitment. Loss of MEIS1 expression has already been shown to be prognostic of worse PrCa and new data shows re- expression of MEIS1 can slow tumor growth in vivo, and lack of MEIS expression in patient tumors is predictive of metastasis and biochemical recurrence. Similarly, HOXB13(G84E) mutation, which we have preliminary data to show breaks interaction between MEIS1 and HOXB13, has also been shown to increase prostate cancer risk and risk of early onset of disease. Thus, when the MEIS1-HOXB13 complex is disrupted by either loss of MEIS1 expression or HOXB13 mutation, the resulting changes to gene targeting and regulation are liable to lead to cancer initiation and progression. However, questions surrounding the sufficiency of disrupting this complex to lead to cancer initiation and/or progression without further oncogenic insults still remain unanswered. Also, a lack of understanding of the transcriptional control exhibited by MEIS1-HOXB13 complex in adult normal prostate and cancer is a clear gap in the field. Given the importance of MEIS1-HOXB13 interactions in prostate development and homeostasis, the known roles for both proteins as tumor suppressors in PrCa, and our data showing HOXB13(G84E) mutation disrupting interaction with MEIS1; We hypothesize that loss of MEIS1- HOXB13 interaction, either by loss of MEIS1 expression or by HOXB13 mutation, is a detrimental event resulting in increased PrCa initiation and metastatic progression; and we seek to identify the transcriptional changes involved in these processes. This innovative proposal makes use of current cutting-edge techniques such as CRISPR/Cas9 for introducing SNPs into the genome, and urogenital sinus mesenchyme (UGSM)recombination, intracardiac injection metastasis models, and next generation sequencing. The studies proposed here define a new paradigm of the importance of the MEIS-HOX axis in adult prostate tissues. Additionally, we expect these studies to be some of the first that point to a definitive and previously unknown mechanism behind the clinically- relevant HOXB13(G84E) mutation that is associated with increased PrCa risk and early onset of disease. This has potential to clearly benefit current PrCa treatment and screening practices, as well as identify new targets for drug development. Finally, the molecular mechanisms identified by this proposal will likely have broader application to other cancer types as well that have been associated with the HOXB13(G84E) mutation.